There is known a surface acoustic wave (SAW) device as one of acoustic wave devices using an acoustic wave. The SAW device is small and light, and is capable of attenuating greatly. Therefore, the SAW device is used in various circuits processing a wireless signal in a frequency band of 45 MHz to 2 GHz in a wireless communication device such as a mobile phone unit or the like. The various circuits are a transmit band pass filter, a receive band pass filter, a local oscillation filter, an antenna duplexer, an IF filter, an FM modulator or the like.
Recently, improvement of frequency-temperature characteristic and downsizing of an acoustic wave device are needed, as performance of a mobile phone unit gets higher and the mobile phone unit is downsized. In order to improve the frequency-temperature characteristic, a silicon oxide film is provided so as to cover a comb-like electrode provided on a surface of a piezoelectric substrate. In order to reduce a device size, there is developed a boundary acoustic wave device in which a dielectric material of which sonic speed is higher than sonic speed in a silicon oxide film is provided on the silicon oxide film covering the comb-like electrode, and acoustic wave energy is confined between the dielectric material and a surface of a piezoelectric substrate.
Frequency variability caused by production tolerance is a common problem among the acoustic wave devices. The frequency is a resonance frequency and an anti-resonance frequency in a case of a resonator, and a center frequency of a pass band in a case of a filter. Various regulation methods are proposed with respect to the frequency variability. For example, International Publication No. 2005-093949 discloses a method for regulating a frequency by adjusting a thickness of a silicon oxide film covering a comb-like electrode and a method for forming a dielectric material on a silicon oxide film covering a comb-like electrode and regulating a frequency by adjusting a thickness of the dielectric material.